534 BELL SYSTEM TECHNICAL JOURNAL 



The value and usefulness of such a large collection of records is 

 dependent very directly upon the accuracy of the test. Therefore 

 considerable attention was given to the calibration of the hearing test 

 equipment and to the evaluation of factors which might afifect the 

 results of the test. There seems little doubt that the records accu- 

 rately portray the hearing characteristics of that section of the popu- 

 lation taking the tests. 



One of the principal objectives of the study was to determine the 

 hearing acuity and the prevalence of defective hearing in the United 

 States population. The visitors who tested their hearing were not a 

 representative sarnple of the population with respect to factors affecting 

 hearing. Consequently a second objective was to determine the 

 relation of hearing to such factors as age, sex, place of residence, 

 economic status, etc. This information is necessary in order to apply 

 the Fair data to the whole population or to specialized groups within 

 the population. 



It is believed that the two important factors, age and sex, have 

 been satisfactorily evaluated. Information on other factors although 

 less complete, is sufficient to justify many applications of the data. 

 In other applications, it is necessary to make reservations and these 

 are described in the text. 



Description of the Test 



The tests were made in sound-insulated rooms arranged to seat 

 seven visitors, each partially screened from the others, as shown in 

 Fig. 1. The test and suitable instructions were recorded on phono- 

 graph records and given through a telephone receiver which the visitor 

 held to his ear. In the musical tone test a pure tone was sounded one, 

 two, or three times, and the listener was instructed to write in a space 

 on a form that was given him the number of times he heard the tone. 

 For a given pitch, nine such sets of tones were sounded, each set 

 fainter than the preceding one. When the tones became too faint to 

 be heard, the listener could not write the number correctly, and thus 

 a measure of his hearing acuity was obtained. This test was made 

 with tones of five different frequencies in the following order: 440, 880, 

 1760, 3520, and 7040 cycles. A typical hearing test record is shown in 

 Fig. 2. 



The correct numbers, which appear in the spaces between the 

 columns of Fig. 2, were printed on the back of the blanks in such a 

 way that they would show through in these spaces when the blank was 

 placed on a brightly illuminated glass shelf. The designations ' ' normal 

 or good," "slightly impaired" and "impaired," which show through 



